I tried the "Fixed header" on Beta Labs (with VE also enabled), and ran into trouble with the big blue Edit button. The little dot on the right has a hover menu to choose between Edit source and VE, but it's very tricky to get the mouse from the blue edit button to the menu links. It usually disappears when I start to move the mouse toward the menu.
Also, on my browsers at least (Iceweasel and Chromium on Debian Testing), the button isn't rendering as intended; there's overlap between the text and the three-dot-menu icon. See http://imgur.com/Vq8Zunf